Ataxia-telangiectasia (A-T) is a progressive genetic disorder affecting the central nervous and immune systems, and involving chromosomal instability, cancer predisposition, radiation sensitivity, and cell cycle abnormalities. Studies of the cellular phenotype of A-T have pointed to a defect in a putative system that processes a specific type of DNA damage and initiates a signal transduction pathway controlling cell cycle progression and repair. For a general review of Ataxia-telangiectasia, reference is hereby made to the review Ataxia-Telangiectasis: Closer to Unraveling the Mystery, Eur. J. Hum. Genet. (Shiloh, 1995) which, along with its cited references, is hereby incorporated by reference as well as to the reviews by Harnden (1994) and Taylor et al (1994).
Despite extensive investigation over the last two decades, A-T has remained a clinical and molecular enigma. A-T is a multi-system disease inherited in an autosomal recessive manner, with an average worldwide frequency of 1:40,000-1:100,000 live births and an estimated carrier frequency of 1% in the American population. Notable concentrations of A-T patients outside the United States are in Turkey, Italy and Israel. Israeli A-T patients are Moroccan Jews, Palestinian Arabs, Bedouins and Druzes.
Cerebellar ataxia that gradually develops into general motor dysfunction is the first clinical hallmark and results from progressive loss of Purkinje cells in the cerebellum. Oculocutaneous telangiectasia (dilation of blood vessels) develops in the bulbar conjunctiva and facial skin, and is later accompanied by graying of the hair and atrophic changes in the skin. The co-occurrence of cerebellar ataxia and telangiectases in the conjunctivae and occasionally on the facial skin--the second early hallmark of the disease--usually establishes the differential diagnosis of A-T from other cerebellar ataxias. Somatic growth is retarded in most patients, and ovarian dysgenesis is typical for female patients. Among occasional endocrine abnormalities, insulin-resistant diabetes is predominant, and serum levels of alpha-fetoprotein and carcinoembryonic antigen are elevated. The thymus is either absent or vestigial, and other immunological defects include reduced levels of serum IgA, IgE or IgG2, peripheral lymphopenia, and reduced responses to viral antigens and allogeneic cells, that cause many patients to suffer from recurrent sinopulmonary infections.
Cancer predisposition in A-T is striking: 38% of patients develop malignancies, mainly lymphoreticular neoplasms and leukemias. But, A-T patients manifest acute radiosensitivity and must be treated with reduced radiation doses, and not with radiomimetic chemotherapy. The most common cause of death in A-T, typically during the second or third decade of life, is sinopulmonary infections with or without malignancy.
The complexity of the disease is reflected also in the cellular phenotype. Chromosomal instability is expressed as increased chromosomal breakage and the appearance in lymphocytes of clonal translocations specifically involving the loci of the immune system genes. Such clones may later become predominant when a lymphoreticular malignancy appears. Primary fibroblast lines from A-T patients show accelerated senescence, increased demand for certain growth factors, and defective cytoskeletal structure. Most notable is the abnormal response of A-T cells to ionizing radiation and certain radiomimetic chemicals. While hypersensitive to the cytotoxic and clastogenic effects of these agents, DNA synthesis is inhibited by these agents to a lesser extent than in normal cells. The concomitant lack of radiation-induced cell cycle delay and reduction of radiation-induced elevation of p53 protein are evidence of defective checkpoints at the G1, S and G2 phases of the cell cycle. The G1 and G2 checkpoint defects are evident as reduced delay in cell cycle progression following treatment with ionizing radiation or radiomimetic chemicals, while the rise in the p53 protein level usually associated in normal cells with radiation-induced G1 arrest is delayed in A-T cells. The defective checkpoint at the S phase is readily observed as radioresistant DNA synthesis (RDS). Increased intrachromosomal recombination in A-T cells was also noted recently. Cellular sensitivity to DNA damaging agents and RDS are usually considered an integral part of the A-T phenotype.
Although these clinical and cellular features are considered common to all "classical" A-T patients, variations have been noted. Milder forms of the disease with later onset, slower clinical progression, reduced radiosensitivity and occasional absence of RDS have been described in several ethnic groups (Fiorilli, 1985; Taylor et al., 1987; Ziv et al., 1989; Chessa et al., 1992). Additional phenotypic variability possibly related to A-T is suggested by several disorders that show "partial A-T phenotype" with varying combinations of ataxia, immunodeficiency and chromosomal instability without telangiectases (12-16) (Ying & Decoteau, 1983; Byrne et al., 1984; Aicardi et al., 1988; Maserati et a;., 1988; Friedman & Weitberg, 1993). Still, other disorders display the A-T phenotype and additional features; most notable is the Nijmegen breakage syndrome that combines A-T features with microcephaly, sometimes with mental retardation, but without telangiectases (Weemaes et al., 1994).
Prenatal diagnoses of A-T using cytogenetic analysis or measurements of DNA synthesis have been reported, but these tests are laborious and subject to background fluctuations and, therefore, not widely used.
A-T homozygotes have two defective copies of the A-T gene and are affected with the disease. A-T heterozygotes (carriers) have one normal copy of the gene and one defective copy of the gene and are generally healthy. When two carriers have children, there is a 25% risk in every pregnancy of giving birth to an A-T affected child.
A-T heterozygotes show a significant excess of various malignancies, with a 3- to 4-fold increased risk for all cancers between the ages of 20 and 80, and a 5-fold increased risk of breast cancer in women. These observations turn A-T into a public health problem and add an important dimension to A-T research, particularly to heterozygote identification. Cultured cells from A-T heterozygotes indeed show an intermediate degree of X-ray sensitivity, but the difference from normal cells is not always large enough to warrant using this criterion as a laboratory assay for carrier detection. The main reason for the unreliability of this assay is the various degrees of overlap between A-T heterozygotes and non-heterozygotes with respect to radiosensitivity. Cytogenetic assays for carriers have the same problems as for prenatal diagnosis, they are labor intensive and not always consistent.
The nature of the protein missing in A-T is unknown. Cell fusion studies have established four complementation groups in A-T, designated A, C, D and E, suggesting the probable involvement of at least four genes or four types of mutations in one gene, with inter-allelic complementation. These four groups are clinically indistinguishable and were found to account for 55%, 28%, 14% and 3% of some 80 patients typed to date. In Israel, several Moroccan Jewish patients were assigned to group C, while Palestinian Arab patients were assigned to group A.
The general chromosomal localization of the putative A-T gene(s) has been determined, but not the sequence. An A-T locus containing the A-T(A) mutations was localized by Gatti et al. (1988) to chromosome 11, region q22-23, using linkage analysis. The A-T(C) locus was localized by applicant to the same region of chromosome 11, region q22-23, by linkage analysis of an extended Jewish Moroccan A-T family (Ziv et al., 1991). Further studies, conducted by an international consortium in which applicant participated (McConville et al., 1990; Foroud et al., 1991; Ziv et al., 1992), reconfirmed this localization in a series of studies and gradually narrowed the A-T locus to an interval estimated at 4 centimorgan, which probably contains also the A-T(E) mutations.
A proposed gene for complementation group D is disclosed in U.S. Pat. No. 5,395,767 to Murnane et al., issued Mar. 7, 1995. This sequence was found not to be mutated in any complementation group of A-T. Further, the gene sequence was mapped physically distant from the presumptive A-T locus.
Therefore, in order to better understand the nature and effects of A-T, as well as to more accurately and consistently determine those individuals who may carry the defective gene for A-T, it would be advantageous to isolate and determine the gene sequence, mutations of which are responsible for causing A-T, and utilize this sequence as a basis for detecting carriers of A-T and thereby be able to more beneficially manage the underlying conditions and predispositions of those carriers of the defective gene.